A method of installing a subsea pipeline having a direct tie-in to a subsea structure includes, during introduction of the pipeline into the sea from a pipe laying vessel, applying a plastic deformation to a region of the pipeline at or close to an end of the pipeline to be tied-in and, either during or following tie-in, elastically deforming the region to increase its radius of curvature.
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1. A method of installing a subsea pipeline having a direct tie-in to a subsea structure, the method comprising the steps of:
during introduction of the pipeline into the sea from a pipe laying vessel, applying a plastic deformation to a region of the pipeline at or close to an end of the pipeline to be tied-in, wherein the plastic deformation creates a radius of curvature in said region of the pipeline when installed that is smaller than the radius of curvature of an adjacent section of the pipeline when installed;
laying the tie-in end of the pipeline on or close to the seabed; and
during tie-in, elastically deforming said region by pulling the tie-in end towards the subsea structure so as to apply a tensile force to the pipeline and increase the radius of curvature of said region.
2. The method according to
3. The method according to
4. The method according to
5. The method according to
6. The method according to
performing said tie-in at the surface;
lowering the tie-in end of the pipeline and the subsea structure to the seabed; and
performing further laying of the pipeline including pulling the pipeline to cause elastic deformation of said region.
7. The method according to
8. The method according to
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The present invention relates the deployment and direct tie-in of subsea pipelines used for the transportation of hydrocarbons.
Pipelines for the transport of hydrocarbons, e.g. oil or gas, are typically laid along the seabed using a laying vessel. Such subsea pipelines can be installed between, for example, two subsea structures, where the subsea structures may be “christmas trees”, riser bases, Blowout Preventers (BOPs), or some other structures. Often one or both ends of the pipeline are connected (or “tied-in”) to a subsea structure using a separate jumper or spool. The extra components and procedures associated with the use of separate jumpers or spools result in high costs for the installation process. Direct tie-in methods can also be used and are often preferable. These methods include:
A typical approach to pipelaying will involve careful design of the subsea structure and of the pipeline configuration in order to ensure that, when laid, the tie-in end of the pipeline is in the correct location and orientation with respect to the connector on the subsea structure. During the direct tie-in process, a very high tensile force is applied to the end of the pipeline, putting the pipeline under tension, in order to bring the end of the pipeline up to the connection point and complete the tie-in process. One of the reasons to put the installed pipe under tension is in order to allow for subsequent thermal expansion of the pipe that can occur during use. Without such tension, a pipeline may buckle as a result of the thermal expansion.
The forces applied to the pipeline during direct tie-in can be very high indeed. This makes high demands of the installation equipment and pipeline structure. Furthermore, at least in the absence of some compensating mechanism, the forces can cause damage to the pipeline and to the connector on the subsea structure.
It is an object of the present invention to overcome or at least mitigate the disadvantages of known direct tie-in procedures. In particular, it is an object of the invention to allow the direct tie-in of a subsea end of a pipeline while minimising the forces on the connection between the pipeline and a subsea structure, minimising the forces required during the direct tie-in process, minimising the stresses on a deflected section of the pipeline near its subsea end, and minimising the area around the subsea structure required to accommodate the deflection of the pipeline.
According to a first aspect of the invention there is provided a method of installing a subsea pipeline having a direct tie-in to a subsea structure. The method comprises, during introduction of the pipeline into the sea from a pipe laying vessel, applying a plastic deformation to a region of the pipeline at or close to an end of the pipeline to be tied-in and, either during or following tie-in, elastically deforming said region to increase its radius of curvature.
As an option, said region may be located within 200 m, and more preferably 100 m, of the tie in end of the pipeline.
As an option, the method may comprise laying the tie-in end of the pipeline on or close to the seabed and pulling the tie-in end towards the subsea structure, said action of pulling resulting in the elastic deformation of said region. Said step of pulling may be achieved using a winch having a winch cable attached to said tie-in end and passing through the subsea structure. Alternatively, said step of pulling may be achieved using a winch having a winch cable extending from the laying vessel or a support vessel and being directly connected to said tie-in end.
As an option, the method may comprise performing said tie-in at the surface, lowering the tie-in end of the pipeline and the subsea structure to the seabed, performing further laying of the pipeline including pulling the pipeline to cause elastic deformation of said region.
As an option, the method may comprise attaching weights and/or buoyancy devices at or close to the tie-in end of the pipeline in order to control the orientation and location of the pipeline.
As an option, the step of applying a plastic deformation to a region of the pipeline at or close to an end of the pipeline to be tied-in may comprise establishing a residual curvature strain of between 0.2% to 0.3%.
As an option, said pipeline may be a steel pipeline.
As described above, a pipeline to be laid on the seabed may be transported on and deployed from a laying vessel. In the case that a substantially inflexible pipeline (for example, steel) is stored on a reel on the laying vessel, it is typically necessary to straighten the pipeline as it is deployed, to remove any residual curvature produced by storing the pipeline on the reel or bending it over the stinger. This is achieved using curvature means that plastically deforms the pipeline to remove the residual curvature.
As described has also been described above, the installation of such straightened pipelines using direct tie-in methods can result in large forces during and following the completion of the connection between an end of a pipeline and a subsea structure, and large stresses in the section of the pipeline near the end of the pipeline. Furthermore, a large area is required for routing the pipeline to the subsea structure, to accommodate the lateral deflection of the pipeline required to align the end of the pipeline with a connection point on the subsea structure. The approach presented here mitigates these problems by using the method of WO02/057674 to create a radius of curvature in a section of the pipeline adjacent to the subsea end of the pipeline (creating a “tie-in and thermal expansion loop”).
In operation, a pipeline will expand under the high pressures and temperatures that can be associated with the transport of, for example, oil or gas. In the case of a generally straight configuration between, for example, two subsea structures that are fixed on the seabed, such thermal expansion (which will result in an increased pipeline length) will result in compressive forces on the pipeline. These compressive forces may be significant and, in the absence of some control mechanism, can cause the pipeline to buckle at unpredictable locations, resulting in the deformation and possible collapse of the pipeline in the horizontal or vertical plane.
In conventional installation methods the pipeline is placed under tension as it is deployed from the laying vessel, due to both the weight of the pipe itself and the forward motion of the laying vessel. This tensile force results in an axial elastic extension in the pipeline, and because the pipeline does not regain its original length before the installation process is complete, the installed pipeline remains under tension. This pre-existing tension in the pipeline mitigates the effects of the longitudinal expansion in the operational pipeline; however, the resulting compression forces may still be large enough to cause buckling. Further measures that are commonly used to protect against the buckling of a pipeline include burying the pipeline in a trench or placing it in an open trench, covering the pipeline with gravel, laying the pipeline along a snaked route, laying the pipeline in a larger casing, and including expansion loops in the pipeline along its length. These methods may be expensive, and may leave uncertainty regarding the likelihood and possible location of buckling in the pipeline.
WO02/057674 aims to mitigate these problems by providing a method for laying a pipeline on the sea bed and that allows for controlled thermal expansion, using thermal expansion loops. This method is illustrated in
The method of WO02/057674 is adapted here to create a tie-in and thermal expansion loop in the section of the pipeline adjacent to the tie-in end of the pipeline.
adjacent to the end of the pipeline 8, the curvature means 7 is adjusted so that a smaller amount of curvature is applied to the opposite side of the pipeline (i.e. the side that shows convex curvature after deployment from the reel and bending over the stringer). In this way, less of the residual curvature produced by the storage on the reel and the bending over the stinger is removed, leaving a radius of curvature in the section of the pipeline section
adjacent to the end of the pipeline that is smaller than a predetermined maximum radius of curvature. This creates a tie-in and thermal expansion loop, at the tie-in end of the pipeline. In a subsequent length of the pipeline L, a radius of curvature greater than a predetermined minimum radius of curvature is produced. In effect, this radius may be infinite, resulting in a completely straightened pipeline section. Subsequent lengths of the pipeline may include thermal expansion loops according to WO02/057674.
In an embodiment of the invention the end of the pipeline—including the tie-in and thermal expansion loop—is directly connected to the subsea structure using a direct pull-in direct tie-in method, as illustrated schematically in
In another embodiment of the invention the end of the pipeline is directly connected to the subsea structure using a deflect-to-connect direct tie-in method, as illustrated schematically in
In another embodiment of the invention the end of the pipeline is directly connected to the subsea structure using a connect and lay-away direct tie-in method, as illustrated schematically in
As a result of the direct tie-in and thermal expansion loops in the region of the tie-in ends of the pipelines, the forces required to pull the pipelines into contact with the respective connectors are greatly reduced. More particularly, the force required to elastically deform the direct tie-in and thermal expansion loop, and thereby stretch the pipeline, is significantly less than the force that would be required to either plastically bend a pipeline into the correct alignment or tension a straight pipeline to increase its length.
A further advantage of providing a direct tie-in and thermal expansion loop in the region of the tie-in end of a pipeline is that this loop also compensates for thermal expansion during use of the pipeline. This is as described in WO02/057674. It may be possible to avoid the need for further expansion loops at midway positions along the pipeline.
It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention.
Endal, Geir, Moen, Johan K., Nes, Rolf Morten, Størkersen, Ståle
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Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
Mar 31 2014 | Statoil Petroleum AS | (assignment on the face of the patent) | / | |||
Sep 30 2016 | STØRKERSEN, STÅLE | Statoil Petroleum AS | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 041443 | /0573 | |
Oct 03 2016 | ENDAL, GEIR | Statoil Petroleum AS | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 041443 | /0573 | |
Oct 03 2016 | NES, ROLF MORTEN | Statoil Petroleum AS | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 041443 | /0573 | |
Feb 07 2017 | MOEN, JOHAN K | Statoil Petroleum AS | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 041443 | /0573 |
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